There were actually plans to do that at one stage - one of the stories they put out was originally written for inclusion in the XXXenophile RPG. But these days they seem to be very busy with Girl Genius and Girl Genius GURPS, so I doubt we'll see it any time soon.

There was also a XXXenophile trading card game; I've never seen it, but I understand it was similar to the Girl Genius card game (non-trading).